Preheat oven to 350 degrees. Beat butter, sugar, egg yolk and vanilla until creamy. Add flour and salt; mix well. Pat dough into a 10x13" rectangle on greased cookie sheet. Bake for about 20 minutes- until dough is set and lightly browned. Remove from oven and immediately top with the Hershey bar squares.
Heat oven to 350°F. Lightly grease cookie sheet or line with parchment paper. Stir together flour, baking soda and salt; set aside. Beat butter, granulated sugar, brown sugar and vanilla in a large bowl until well blended. Add eggs; beat well. Gradually add flour mixture, beating until well blended. Stir in toffee bits.
Remove from oven and immediately top with the Hershey bar squares. When squares have softened (and it doesn't take long!) spread evenly and top with the finely chopped walnuts.